Transporters play a particular role in the functioning of an organism. On the one hand they determine the uptake or emission of a substance into or out of a cell or an organism, on the other they control the transport and distribution of substances between the cells. As a rule transporters lie at the beginning or the end of a metabolic pathway and thereby take charge of fundamental higher controlling functions.
Purine and pyrimidine bases and the nucleosides and nucleotides derived from them are numbered among the transporter metabolites which serve as the building blocks of the nucleic acids. The uptake of these substances, for example during pollen fertilization and the early development of the embryo in germinating seeds, has an important physiological significance in the preparation of early stages for the synthesis of nucleic acids. As phytohormones, cytokinins are structurally closely related to the purine bases and the purine nucleosides. These phytohormones regulate many processes during plant development. Very little is known about the origin of these hormones, yet their effective transport in the plant is of decisive importance.
In bacteria nuclear base transport systems for adenine, cytosine and uracil were characterized and the corresponding genes could be cloned. In the baker's yeast Saccharomyces cerevisiae three differing active transport systems for nucleosides and nuclear bases have so far been well characterized both genetically and physiologically. Nucleoside transport systems have been described and characterized in a number of mammalian cells. Besides these nucleoside transporter systems specific transporter systems for nuclear bases have also been described.
In higher plants transport processes for distribution of assimilates, metabolites and phytohormones are of critical physiological importance. Only very little is yet known about the transport of nuclear bases and their derivatives in plants, and up till now in contradistinction to bacteria, fungi and mammals only a few transport systems for these substances have been described. Clarification of the course of events of nuclear base transport in plants, giving similarly detailed information as that put in place for other organisms, is not available and owing to the difficulty of molecular biological analysis in plants, is scarcely practicable. Nevertheless, there exists great interest in the identification and characterization of plant genes coding for nuclear base transporters or transporters for chemically related substances. In addition on account of the central function of transporters a great interest exists in plants, which are in a position to transport great quantities of nuclear bases and their derivatives, as well as in the provision of possibilities for altering the distribution of nuclear bases in transgenic plants and mutants.